1. Field of the Invention
The invention relates, generally, to a working fluid circuit for an internal combustion engine, and more specifically, to a fluid circuit for a turbocharged internal combustion engine that employs exhaust gas recirculation.
2. Description of the Related Art
Exhaust gas recirculation is commonly employed in connection with internal combustion engines as a means for controlling the generation of oxides of Nitrogen (NOx) generated during the operation of the engine. This involves the recirculation of exhaust gas byproducts, typically taken from the exhaust manifold, and routed into the intake air supply of the engine. The exhaust gas reintroduced into the engine cylinder in this way reduces the concentration of oxygen in the fuel/air mixture. A reduction of oxygen in the fuel/air mixture results in a lower maximum combustion temperature and slows the chemical reaction of the combustion process. This decreases the formation of nitrous oxides (NOx) that are discharged from the engine. In addition, the exhaust gases often contain a portion of unburned hydrocarbon that, left uncombusted, forms a part of the exhaust emissions generated during the operation of any given internal combustion engine. However, when the unburned hydrocarbons are recirculated back to the combustion chamber, they are burned thereby further reducing the emission of undesirable exhaust gas byproducts from the engine. In view of the benefits derived by employing this technique, exhaust gas recirculation is commonly found in connection with both spark ignition and compression ignition (diesel) engines. Exhaust gas recirculation is particularly useful in connection with internal combustion engines used in motor vehicles, such as passenger cars, light duty trucks, and other motorized equipment.
Turbochargers are also known to be used in the related art to provide charge air to the working fluid circuit of an engine. More specifically, when a engine is turbocharged, the pressurized exhaust gas acts on a turbine that, in turn, drives a compressor. The compressor pressurizes the intake air for the internal combustion engine making it more dense. Dense intake air improves combustion resulting in increased power from the engine. Turbochargers are employed in connection with both spark ignition and compression ignition (diesel) engines for this purpose.
In addition to recirculating the exhaust gases, it is also known in the related art that lowering intake manifold temperatures reduces the formation of nitrous oxides generated as a product of combustion. However, the exhaust gases that are available for recirculation are generally very hot, sometimes exceeding 550 C. Thus, it is known in the art to cool the recirculated exhaust gas in order to lower the intake air temperature thereby further reducing the production of NOx where exhaust gas recirculation is employed. In addition, it is also known to cool the charge air delivered by the turbocharger prior to induction into the combustion chamber. The EGR intercooler and charge air cooler are separate heat exchangers that are employed to cool these two engine working fluids. One example of a turbocharged internal combustion engine having intercooled exhaust gas recirculation is found in U.S. Pat. No. 6,116,026, issued Sep. 12, 2000 and assigned to the assignee of the present invention. The disclosure of this patent is incorporated herewith.
In turbocharged internal combustion engines, the exhaust gas to be recirculated is generally removed upstream of the turbine, routed through the intercooler, and then reintroduced into the intake air stream downstream of the compressor and the charge air cooler. Exhaust gas intercoolers of this type often employ engine coolant as the cooling medium. While these coolers have generally worked for their intended purpose in the past, disadvantages still remain. More specifically, using the engine coolant as the cooling medium increases the heat load on the engine cooling system and thereby necessitates larger vehicles radiators. The use of multiple or staged coolers has also been suggested in the prior art, but this only adds to the bulk of the engine and tends to overcomplicate the engine cooling system. Furthermore, the extreme temperature differentials that exist between the exhaust gas and the coolant in the intercooler creates a harsh working environment. Some products of combustion found in the exhaust gas are highly corrosive and can condense at certain operating temperatures within the intercooler. These harsh operating environments and corrosive condensate can cause the liquid to air intercoolers to leak over time.
Accordingly, there is a need in the art for an engine working fluid circuit that is capable of cooling both the recirculated exhaust gas and the charge air without the addition of multiple coolers. Furthermore, there is a need in the art for an engine working fluid circuit that can cool the recirculated exhaust gas and charge air without the disadvantages associated with leaks at the liquid/air cooling interface. Finally, there remains a need in the art for an engine working fluid circuit that employs an overall simpler cooling strategy, eliminates redundant components, and that improves reliability for the overall system.
The present invention overcomes the disadvantages of the related art in working fluid circuit for a turbocharged internal combustion engine that employs exhaust gas recirculation. The working fluid circuit of the present invention includes an exhaust gas passage through which exhaust gas under pressure flows from the internal combustion engine, a charge air passage, and a turbocharger. The turbocharger is operatively driven by the exhaust gas flowing from the internal combustion engine and acts to provide pressurized air to the charge air passage. The engine working fluid circuit further includes an exhaust gas recirculation passage that extends between the exhaust gas passage and the charge air passage, but bypasses the turbocharger. The exhaust gas recirculation passage thereby provides a path for recirculating a predetermined amount of exhaust gas into the charge air passage in such a way that the exhaust gas and the charge air are mixed together. The working fluid circuit further includes an intake passage that provides intake air to the internal combustion engine and a single charge air cooler. The single charge air cooler operatively interconnects and provides fluid communication between the charge air passage and the intake passage. Furthermore, the single charge air cooler acts to cool the mixed charge air and the recirculated exhaust gas prior to induction into the internal combustion engine through the intake passage.
One advantage of the working fluid circuit of the present invention is that it is capable of cooling both the recirculated exhaust gas and the charge air without the addition of multiple coolers. Still another advantage of the present invention is that it can cool the recirculated exhaust gas and charge air without the disadvantages associated with leaks the occur in heat exchangers employing a liquid/air cooling interface. Still another advantage of the present invention is that by mixing the very hot exhaust gas with the much cooler charge air upstream of the charge cooler, the gas temperature entering the cooler is substantially lower (xcx9c300xc2x0 C. versus  greater than 550xc2x0 C.), thus avoiding the problem of thermal fatigue stress cracking, which has been experienced with separate EGR gas/liquid coolers. Still another advantage of the present invention is that the recirculated exhaust gas is more thoroughly mixed with the charge air by being introduced upstream of the charge air cooler. Furthermore, the working fluid circuit of the present invention employs an overall simpler cooling strategy, eliminates redundant components, and improves the reliability of the overall system.